Planar light emitting devices are drawing attention including those using an organic electroluminescence element (The “electroluminescence” may be hereinafter referred to as EL). Since the organic EL element is capable of emitting a light which is white or near white, the planar light emitting device prepared by using such organic EL element can be used for the light source of lighting apparatus and backlight of displays.
Known organic EL elements are composed of components including an organic EL layer, a transparent electrode, and a light-transmitting substrate made of a glass or polymer film, and those elements suffer from non-negligible loss of light when the light emitted by the organic EL layer is extracted to the exterior through the light-transmitting substrate. The efficiency of extracting the emitted light to the exterior (hereinafter referred to as light extraction efficiency) is currently as low as about 20%. In addition, since a white organic EL element is commonly provided with a plurality of light-emitting elements each emitting red, green, and blue colors or yellow and blue colors and refraction and reflection behaviors of the light in each component are different by the wavelength, change in the color due to the difference in the viewing angle (hereinafter referred to as color deviation) is an issue to be solved.
In addition, glass is mainly used for the light-transmitting substrate of the current organic EL element, and the glass has the risk that broken pieces of the glass scatter by the impact at the breaking of the glass.
Exemplary methods employed to improve the light extraction efficiency of the organic EL element include provision of a resin layer containing a light-scattering material (Japanese Patent Application Laid-Open No. 2003-109747) and provision of a light-scattering pressure sensitive adhesive layer (Japanese Patent Application Laid-Open No. 2010-218738) on the surface of the light-transmitting substrate opposite to the side of the light emitter. For the improvement of the light extraction efficiency and reduction of the color deviation as well as prevention of the scattering of the broken light-transmitting substrate, the methods proposed include provision of a particle-containing resin layer between the transparent electrode and the light-transmitting substrate simultaneously with the provision of a lens sheet on the light exiting surface (Japanese Patent Application Laid-Open No. 2011-86527).
However, in the method described in Japanese Patent Application Laid-Open No. 2003-109747, the function of preventing the scattering of the broken pieces of the light-transmitting substrate was lacking, or prevention of the color deviation was insufficient.
In the method of using the light-scattering pressure sensitive adhesive layer described in Japanese Patent Application Laid-Open No. 2010-218738, the light extraction efficiency was insufficient and prevention of the color deviation was also insufficient.
In the method described in Japanese Patent Application Laid-Open No. 2011-86527, a transparent electrode having a thickness of several dozen nm was formed on the resin layer which was already provided with a large surface irregularity due to the light scattering particles, and such formation of the electrode was unrealistic considering the current production technology.
It could therefore be helpful to provide a planar light emitting device with a high light extraction efficiency, a small color change by the viewing angle and, also, with a less scattering of broken pieces when the light-transmitting substrate is broken. It could also be helpful to provide a front film used for the planar light emitting device.